1. Field of the Invention
The present invention relates to a lithography method, and more particularly, to a lithography method utilizing an alternate phase shift mask (alt PSM) to eliminate a side-lobe effect and to form patterns having a superior resolution in a photoresist layer.
2. Description of the Prior Art
In integrated circuit making processes, a lithographic process has become a mandatory technique. In a lithographic process, a designed pattern, such as a circuit pattern, a doping pattern, or a contact hole pattern is created on one or several photo masks, then the pattern on the photo mask is transferred by light exposure, with a stepper and scanner, into a photoresist layer on a semiconductor wafer. Only by using a lithographic process can a wafer producer precisely and clearly transfer a complicated circuit pattern onto a semiconductor wafer.
It is an important issue for solving resolution of the lithographic process due to the reducing device sizes of the semiconductor industry. Theoretically, using short wavelengths of light to expose a photoresist layer will improve the resolution right away. Short wavelengths of light are desirable as the shorter the wavelength, the higher the possible resolution of the pattern. This method, though it seems simple, is not feasible. First, light sources for providing short wavelengths of light are not accessible. Secondly, the damage of equipment is very considerable when short wavelengths of light is used to expose a photoresist layer, leading to a shorted equipment lifetime. The cost is thus raised, which makes products not competitive. Due to the conflicts between theory and practice used in manufacturing, the manufacturers are all devoted to various researches so as to get over this problem.
Please refer to FIG. 1. FIG. 1 is a schematic diagram illustrating a method for improving a resolution of contact hole patterns 22 by utilizing assist features according to the prior art. As shown in FIG. 1, the prior art method for improving the resolution of the contact hole patterns 22 by utilizing assist features is to proceed an optical proximity correction (OPC) of a photo mask pattern first. That means, some small assist bars (not shown) are inserted into a regular layout pattern according to results of simulation when outlaying a mask (not shown). Then, the mask layout (not shown) is transferred onto the photo mask 10 after a verify procedure is proceeded. Except for a plurality of contact hole features 12, a plurality of assist features 14 inserted between contact hole features 12 are included on the photo mask 10.
An exposure process is thereafter performed to irradiate the photo mask 10 with light so that a plurality of contact hole patterns 22, corresponding to the contact hole features 12, are formed in a photoresist layer 18 on a surface of the wafer 16. Since the assist features 14 inserted between the contact hole features 12 are merely used for improving the resolution of each of the contact hole patterns 22 by applying light interference principle, their corresponding patterns are not formed in the photoresist layer 18 after the exposure process. Therefore, the photoresist layer 18 is smooth, except for the contact hole patterns 22 to allow contact holes (not shown), to be formed in a subsequent etching process.
In addition to the prior art method for improving the resolution of the contact hole patterns by utilizing assist features, another method involves the use of a half-tone mask. The half-tone mask is a kind of phase shift mask. Please refer to FIG. 2. FIG. 2 is a schematic diagram illustrating a prior art method for improving a resolution of contact hole patterns 44 by utilizing a half-tone mask. As shown in FIG. 2, the half-tone mask 30 is formed from a quartz substrate. A plurality of contact hole features 32 and a not completely transparent region 34 enclosing each of the contact hole features 32 are included on the half-tone mask 30. Actually, the not completely transparent region 34 of the half-tome mask 30 is coated with a single-layered embedded layer 36. The embedded layer 36 is not only an absorption layer, but also is a phase shift layer. Since the transmittance and the phase shift angle required by the process can be achieved by utilizing the single-layered embedded layer 36, the embedded layer 36 is also called an absorptive shifter.
When a specific wavelength of light (correlating to the embedded layer) is utilized to perform the exposure process, light will pass through each of the contact hole features 32 successfully to reach a photoresist layer 42 on a wafer 38. Because the embedded layer 36 has a specific transmittance (usually between 2˜15%), portions of the light passing through the embedded layer 36 will have a phase shift of 180 degrees relative to the original light, and resulting in destructive interference with the light passing through the contact hole features 32. After a develop and a bake process are performed, the contact hole patterns 44 corresponding to the contact hole features 32 will be formed in the photoresist layer 42. Thanks to the interference effect caused by light passing through the embedded layer 36, the resolution of the contact hole patterns 44 is improved.
However, both of the above-mentioned methods have limitation, in application or in effect. When the assist features are utilized to improve the resolution of the contact hole patterns, the resolution is not improved if the patterns to be formed are too small. Moreover, the contact hole patterns, that should be separated from each other, may connect together after the lithography process. In fact, a ratio of the spacing between the patterns to be formed to a line width of the patterns to be formed must be greater than ½ when using this method. When utilizing the half-tone mask to improve the resolution of the contact hole patterns, the light intensity of the side-lobe is too high to produce extra patterns if the transmittance of the embedded layer is too high. If the transmittance of the embedded layer is too low, the destructive interference of light caused by the phase shift is not enough. The side-lobe phenomenon at the edge of patterns cannot be eliminated, leading to the unsatisfied pattern edge resolution.
In addition, some methods involve adjusting equipment parameters, such as changing exposure energy, changing exposure time, adjusting aperture, and utilizing off-axis illumination method, have been adapted in order to achieve a better resolution and make a compromise between resolution and depth of focus. However, satisfied results are not obtained. At the same time, manpower and time are wasted and equipment is damaged. Therefore, it is very important to develop a lithography method to improve the resolution of the contact hole patterns effectively. This method is able to be applied to small-sized patterns, and should not damage equipment when using the current equipment.